Cathode ray gun having first and second grids with orthogonal apertures

ABSTRACT

A cathode-ray tube having an electron gun comprising a cathode, a first grid, a second grid a third grid, and a focusing lens which focuses the electron beam on the screen. The first grid has an elongate aperture the long axis of which is parallel to the vertical scanning direction. The second grid has an aperture the long axis of which is a right angles to the long axis of the aperture in the first grid. The aperture in the second grid is widened in the direction of the screen and its dimension and its distance from the first grid are adjusted for intercepting at least 5 percent of the beam current so as to reduce the aberrations.

I United States Patent I191 [111 3,866,081 Hasker et a1. 1 1 Feb. 11, 1975 1 CATHODE RAY GUN HAVING FIRST AND 3.524.094 8/1970 Hasker et al 313/76 iggggg WITH ORTHOGONAL FOREIGN PATENTS OR APPLICATIONS 100.660 4/1937 Australia 313/83 [75] Inventors: Jan Hasker;.lac0bus Johannes 467,293 6/1937 Great Britain 313/86 Maria Joseph De Klerk, both of Emmasmgel' Emdhoven Primary Examiner-Robert Segal Netherlands Attorney, Agent, or Firm-Frank R. Trifari; George B. [73] Assignee: U.S. Philips Corporation, New Berk?! York, NY. [22] Filed: June 21, 1973 [57] ABSTRACT [21] A p], N(),j 372,402 A cathode-ray tube having an electron gun comprising a cathode, a first grid, a second grid a third grid, and a focusing lens which focuses the electron beam on [301 Fore'gn Apphcat'on Pnonty Data the screen. The first grid has an elongate aperture the June 24, i972 Netherlands 7208728 long axis of is parallel to the ertical scanning a direction. The second grid has an aperture the long U-S. Cl. axis of is a angles to the long axis of the Int. Cl. aperture in the first The aperture in the second Field Of Search 313/82 82 C, 83, 86, grid is widened in the direction of the screen and its 313/87 dimension and its distance from the first grid are adjusted for intercepting at least 5 percent of the beam References Clted current so as to reduce the aberrations.

UNITED STATES PATENTS 5 Cl i 2 D 2,884,559 4/1959 Cooper et a1 313/86 x 3 rawmg 2 5 r-- -r J CATHODE RAY GUN HAVING FIRST AND ND DS IIiQRTHOGQNA APERTURES The invention relates to a device comprising a cathode-ray tube, which cathode-ray tube comprises an astigmatic electron gun having centered along an axis: a cathode, a first grid, a second grid and a third grid for producing an electron beam, a focusing lens and a target on which the electron beam impinges with a narrow elongate spot, said first grid having a narrow elongate aperture the length direction of which is substantially parallel to the length direction of the spot which the electron beam causes in the center of the target, so that an astigmatic lens is present in the electron gun for producing a structure of the electron beam in which same is focused in a point at some distance in front of the target on the side of the electron gun with respect to electron paths in an imaginary plane through the length direction of the spot in the center of the target and the said axis and is focused substantially on the target with respect to electron paths in a plane through the said axis and at right angles to the length direction of the spot in the center of the target, said second grid having an aperture which. on the side remote fromthe cathode, widens in the direction of the target in planes parallel to the plane through the said axis and the length direction of the spot in the center of the target. The invention also relates to a cathode-ray tube for such a device.

Such a device comprising a cathode-ray tube is known from the Published Dutch patent application Nr. 7,0] 1,41 l. in the cathode-ray tube described in this patent application, the aperture in the second grid widens in the direction of the target and in planes at right angles to the length direction of the aperture so as to reduce the aberrations in the electron beam in said planes. These aberrations arise partly without the said widening of the aperture by a considerable curvature of the equipotential lines in the proximity of the aperture. The result of the aberrations is that the elongate electron spot on the screen upon deflecting the electron beam obtains an S-shaped deformation, as a result of which the effective width of the spot becomes larger. in the Dutch patent application Nr. 7,01 1,411 it is described how said aberrations can be reduced. When the cathode-ray tube is a so-called indexing tube, the elongate spot is rotated as a result of deflection errors so that, with large currents, the endexing system no longer operates. As a matter of fact, several indexing lines are then hit simultaneously. This can be mitigated by using dynamic correction means for spot rotation. Of course, this is not very effective in the case of a spot distorted in the form of an S.

It is the object of the present invention to provide a device having a cathode-ray tube in which means for dynamic correction of the rotation of the spot which is formed by the deflection need not be used.

According to the invention a device of the type mentioned in the first paragraph is characterized in that at least 5 percent of the electron beam intercepted by the second grid. The invention is based on the recognition of the fact that the spherical aberration in the direction of the long axis of the spot still contributes to the width of the spot with deflected beam and that furthermore the length of the spot with increasing beam cur'rentincreases because the dimensions of the effectively emitting cathode surface measured paralles to the length direction of the aperture in the first grid increase. If the length and the width of the spot with deflected beam can be restricted, it is possible to omit compensation of the said rotation of the spot. Restriction of the said length can be effected by restricting the said dimension of the emitter cathode surface. Because the said aberrations are mainly formed in the immediate proximity of the cathode, this would also favourably influence the said width. A restriction of the said dimensions of the effectively emitting cathode surface to, for example percent of the original value, only has a small influence on the total beam current but producesa considerable reduction ofthe aberrations since it is just the edge rays which are strongly subject to aberrations. A restriction of the effectively emitting surface of the cathode by making said surface partly inactive, however, is a structurally difficult measure, because such a cathode is difficult to manufacture and because in that case the center of the active surface has to be accurately aligned with the remaining electrodes of the gun. Another known method is the interception of the edge rays with a diaphragm in a field-free space, for example, a diaphragm in the first electrode of the focusing lens, but this measure also results in structural difficulties in connection with the alignment. Moreover, said measure would in this case not produce the desired result because in the present case the edge rays with aberrations already intersect the electron paths situated closer to the axis of the gun in the region between the cathode and the diaphragm so that many of them nevertheless can pass the diaphragm. On the contrary, the solution of the invention technologically is very simple and is enabled by the very small radius of curvature of the equipotential lines on the side of the second grid remote from the cathode, which the electron gun in a device of the type mentioned in the first paragraph has, since otherwise the passed rays which travel immediately along the edge of the aperture in this second grid would again obtain aberrations due to a considerable curvature of the equipotential lines. The desired diaphragming is obtained by reducing the dimension in the longitudinal direction of the spot with nondeflected beam, of the part of the aperture in the second grid present on the cathode side and, possibly, the distance between the first and the second grid. It is even possible to choose the geometry so that the sperical aberration which remains in the said direction for the passed part of the beam is smaller than for the corresponding part of the beam in an electron gun without the said diaphragming. This latter is a result of the fact that in the said direction the field in the proximity of the cathode is more homogeneous than in a gun without the said diaphragming. It is furthermore more pointed out that,

as a result of the effect of the above curvature of the equipotential lines, the interception of beam current by one of the electrodes of the electron gun under the influence of which the beam cross-over is formed, is an unusual measure in television display tubes.

A device comprising a cathode-ray tube according to the invention is preferably constructed so that for a current strength on the target. of 3 X (D/635) X (25volt) mA, where D is the length of the diagonal of the target in mms and V is the potential of the target relative to the cathode in kV, at least 5 percent of the electron beam is intercepted by the second grid. Preferably, at most 40 percent of the electron beam is inter cepted by the second grid. Furthermore, the device is preferably made so that the aperture in the second grid on the side of the first grid in planes at right angles to the axis of the electron gun has a dimension which, measured parallel to the length direction of the aperture in the first grid, is smaller than half of the dimension of the aperture in the first grid measured in the same direction.

The apertures in the first grid and the aperture in the second grid on the side of the first grid preferably have an elliptic cross-section.

Very favourable dimensions for the electron gun are substantially so that the gap between the cathode and the first grid is equal to 85a, the thickness of the first grid is equal to 50a, the gap between the first grid and the second grid is equal to 225a, the thickness of the narrowest part of the aperture in the second grid is equal to 200a, the aperture in the first grid has a long axis of 6900 and a short axis of 230a and the aperture in the second grid on the side of the first grid has a long axis of 69011 and a short axis of 230a, where a is any length unit. The length unit a is preferably 0.001 mm.

The invention will be described in greater detail, with reference to the accompanying drawing of an embodiment, of which FIG. I shows a cathode ray tube for a device according to the invention and I FIG. 2 shows the electron gun of said tube.

The cathode-ray tube shown in FIG. 1 is a display tube for colour television of the type having one electron gun and indexing strips on the screen for positioning the electron beam, that is to say, a so-called indexing tube. The tube is a so-called l l-25 inches tube, which means that the electron beam has to be deflected through 55 to reach the corners of the screen and the diagonal of the screen is 635 mm long. The tube comprises an evacuated glass envelope 1 having an electron gun 2 and a display screen 3. Deflection coils 4 serve to scan the display screen 3 with an electron beam produced by the electron gun 2. A focusing lens 5 focuses the electron beam on the display screen 3. The electron gun 2 comprises a cathode 6, a first grid 7, a second grid 8 and a third grid 9. The focusing lens 5 comprises a first electrode 10 which forms one assembly with the third grid 9, a second electrode 11 and a third electrode 12. The display screen 3 comprises a luminescent layer 13 consisting of a vertical phosphor strip separated by black inactive strips, a thin aluminium layer 14 which reflects light from the luminescent layer but is permeable to electrons and a layer 15 having indexing strips consisting of a phosphor which emits ultraviolet light under the influence of an electron beam. The ultraviolet light is intercepted by aphoto-multiplier tube-(not shown) outside the envelope 1, the supplied signal of which serves for the positioning of the electron beam relative to the indexing stripsand hence relative to the phosphor strips. This is effected in known manner which is of no significance for the invention. In order to obtain a picture having an acceptable structure, the width of the phosphor strips and the black strips is approximately 0.2 mm.

The factors by which the colour reproduction in the indexing system is determined are mentioned in the paper Astigmatic gun for the beam indexing color television display in IEEE Transactions on Electron Devices" vol. ED-l8, No. 9, September, 1971. it is inter alia of importance that the dimension of the spot in the direction at right angles to the phosphor strips be of the same order of magnitude as the width of the phosphor strips. It is explained in the said paper that it is favourable for this purpose and in order to minimize deflection errors that the beam be astigmatic. In order to avoid phosphor saturation at high beam currents as a result of which the color reproduction would be adversely influenced, the spot must be elongate in which of course the dimension of the spot in the direction of the phosphor strips must nevertheless be sufficiently small to satisfy requirements with respect to the resolving powder in a vertical direction. It is furthermore explained in the said paper that, in order to satisfy the above mentioned requirements, it is favorable to produce the beam by means of an astigmatic electron gun in which the focusing of the said beam on the target is produced by means of a rotationally symmetric focusing lens.

FIG. 2 shows the electron gun 2 and the focusing lens 5 with their already specified electrodes. The first grid 7 comprises an elliptic aperture 16 the long axis of which lies in the plane of the drawing and has a length of 0.69 mm. The short axis of the aperture 16 is at right angles to the plane of the drawing and has alength of 0.23 mm. The second grid 8 comprises an elliptic aperture 17 the short axis of which lies in the plane of the drawing and has a length of 0.23 mm. The long axis of the aperture 17 is at right angles to. the plane of the drawing and has a length of 0.69 mm. The aperture 17 develops into a rectangular aperture 18 having a width at right angles to the plane of the drawing of 5 mm and a height in the plane of the drawing of 1 mm. Measured parallel to the axis 19 of the electron gun the dimensions are as follows: the length of the gap between the cathode 6 and the first grid 7 is 0.085 mm; the length of the aperture 16 of the first grid 7 is 0.050 mm. The length of the gap between the first grid 7 and the sec- 0nd grid 8 is 0.225 mm; the length of the aperture 17 in the second grid 8 is 0.200 mm and the length of the aperture 18 is 0.450 mm; the length of the gap between the second grid 8 and the third grid 9 is 3 mm; thedistance between the cathode and the center of the focusing electrode 1 1 is 60 mm. It is to be noted that, besides by the geometry of the electron gun, the las.tmentioned distance is determined by the dimensions of the target and the type of focusing lens used. By the elliptic apertures in the first grid 7 and the second grid 8 an astigmatic electron beam is formed and that in such manner that forelectron paths in a plane through the axis 19 and at right angles to the plane of the drawing a first beam cross-over is formed between the first grid 7 and the second grid-8. Also under the influence of the focusing lens 5- a second beam cross-over approximately in the center of the second electrode 1 l of the focusing lens 5 is formed for electron paths in the plane of the drawing. These beam cross-overs are focused by the focusing lens in such manner that the first beam cross-- over is displayed on the display screen 3 and forms the short axis of the spot and the electron paths diverging from the second beam cross-over form the long axis of the spot.

The height of the aperture 17 (in the plane of the drawing and at right angles to the axis 19) is only 0.23

mm as a result of which the beam begins to be intercepted by the second grid for beam currents exceeding 1.5 to 2 mA and at 3 mA beam current approximately 10 to 15 percent of the beam current is intercepted by the second grid. The intercepted part of the beam current is formed by the edge rays with most aberrations. It has been found that, without the use of dynamic correction means for spot rotation upon deflection, the achieved gain in brightness in percent compared with the configuration described in the Dutch patent application nr. 7,011,411 also operated without dynamic spot rotation is much larger than the said intercepting percentages. Of course it is also possible to use dynamic spot rotation in addition to the present invention.

Of course the given dimensions are only meant by way of example. It is possible, for example, to multiply all the dimensions of the electron gun by the same factor. Good results may be expected with a gap between the cathode 6 and the first grid 7 of between 0.050 and 0.200 mm, a thickness of the first grid 7 between 0.030 and 0.200 mm, a gap between the first grid 7 and the second grid 8 between 0.100 and 0.500 mm, a depth of the aperture 17 between 0.075 and 0.500 mm, a depth of the aperture 18 between 0.200 and 2.0 mm (measured parallel to the axis 19), and a gap between the second grid 8 and the third grid 9 between 1.5 and 6.0 mm. As already noted the distance between the cathode and the focusing lens is also determined by the geometry of the electron gun.

The electrodes of the cathode-ray tube shown by way of example have, in the operating tube, substantially the following voltages relative to the cathode 6.

the first grid 7: between -200 and -70 V the second grid 8: 1.3 kV

the third grid 9 the first electrode 10 25 kV the second electrode 1 l: 7 kV the third electrode 12'. 25 kV.

In addition, the potential of the display screen 3 is equal to 25 kV.

Of course, the use of an electron gun according to the invention is not restricted to an indexing color display tube. Notably, a gun which produces an a stigmatic beam having the described structure will give good satisfaction for a shadow mask tube having three beams with their axes in one plane (it is to be noted that in the above the focusing voltage is adjusted so that the beam in the horizontal plane is focused substantially on the screen). lt is also possible to use an electron gun according to the invention in a shadow mask tube so as to avoid moire patterns in the picture. Since said patterns occur in particular at low beam currents, the vertical dimension of the spot in the case of a low beam current must be sufficiently large, on the other hand the said dimensions at high beam current may not be too large so as to obtain a sufficient vertical definition. This means that the beam must already begin to be intercepted by the second grid at comparatively low currents. It is to be noted, however, that, compared with the electron guns conventionally used in shadow mask tubes, the cathode load is increased by it.

What is claimed is:

1. A cathode-ray tube comprising an astigmatic electron gun having centered along an axis: a cathode, a first grid, a second grid and a third grid for producing an electron beam, a focusing lens and a target upon which the electron beam impinges with a narrow elongate spot, said first grid having a narrow elongate aperture the length direction of which is substantially parallel to the vertical scanning direction, so that an astigmatic lens is present in the electron gun for producing a structure of the electron beam in which the same is focused in a point at some distance in front of the target on the side of the electron gun with respect to electron paths in an imaginary plane through the length direction of the spot in the center of the target and the said axis, and is focused substantially on the target with respect to electron paths in a plane through the said axis and at right angles to the length direction of the spot in the center of the target, said second grid having an aperture which, on the side remote from the cathode, widens in the direction of the target and is elongated at substantially right angles to that of said first grid. said aperture having a dimension which, measured parallel to the direction of the vertical scanning, is smaller than half of the dimension of the aperture in the first grid measured in the same direction, said third grid having an aperture exceeding in size the apertures of said first and second grids, the dimension of the aperture of said second grid in the vertical scanning direction and the distance between the first and second grid being adjusted for intercepting at least 5% of the electron beam by the second grid.

2. A cathode-ray tube as claimed in claim 1, wherein for a current strength on the target of 3 (D/635) (25/V)mA, where D is the length of the diagonal of the target in mms and V is the potential of the target relative to the cathode in kV, at least 5 percent and at most 40 percent of the electron beam is intercepted bythe second grid.

3. A cathode-ray tube as claimed in claim 1, wherein the aperture in the first grid and the aperture in the second grid on the side ofthe first grid havean elliptic cross-section.

4. A cathode ray tube as claimed in claim 1, wherein the gap between the cathode and the first grid is substantially equal to a, the thickness of the first grid is substantially equal to 500, the gap between the first grid and the second grid is substantially equal to 2250, the thickness of the narrowest part of the aperture in the second grid is substantially equal to 200a, the aperture in the first grid has a long axis ofsubstantially 690a the length unit a is equal to 0.001 mm. 1: 

1. A cathode-ray tube comprising an astigmatic electron gun having centered along an axis: a cathode, a first grid, a second grid and a third grid for producing an electron beam, a focusing lens and a target upon which the electron beam impinges with a narrow elongate spot, said first grid having a narrow elongate aperture the length direction of which is substantially parallel to the vertical scanning direction, so that an astigmatic lens is present in the electron gun for producing a structure of the electron beam in which the same is focused in a point at some distance in front of the target on the side of the electron gun with respect to electron paths in an imaginary plane through the length direction of the spot in the center of the target and the said axis, and is focused substantially on the target with respect to electron paths in a plane through the said axis and at right angles to the length direction of the spot in the center of the target, said second grid having an aperture which, on the side remote from the cathode, widens in the direction of the target and is elongated at substantially right angles to that of said first grid. said apertuRe having a dimension which, measured parallel to the direction of the vertical scanning, is smaller than half of the dimension of the aperture in the first grid measured in the same direction, said third grid having an aperture exceeding in size the apertures of said first and second grids, the dimension of the aperture of said second grid in the vertical scanning direction and the distance between the first and second grid being adjusted for intercepting at least 5% of the electron beam by the second grid.
 2. A cathode-ray tube as claimed in claim 1, wherein for a current strength on the target of 3 X (D/635)2 X (25/V)mA, where D is the length of the diagonal of the target in mms and V is the potential of the target relative to the cathode in kV, at least 5 percent and at most 40 percent of the electron beam is intercepted by the second grid.
 3. A cathode-ray tube as claimed in claim 1, wherein the aperture in the first grid and the aperture in the second grid on the side of the first grid have an elliptic cross-section.
 4. A cathode ray tube as claimed in claim 1, wherein the gap between the cathode and the first grid is substantially equal to 85a, the thickness of the first grid is substantially equal to 50a, the gap between the first grid and the second grid is substantially equal to 225a, the thickness of the narrowest part of the aperture in the second grid is substantially equal to 200a, the aperture in the first grid has a long axis of substantially 690a and a short axis of substantially 230a and the aperture in the second grid on the side of the first grid has a long axis of substantially 690a and a short axis of substantially 230a, where a is an arbitrary length unit.
 5. A cathode-ray tube as claimed in claim 4, wherein the length unit a is equal to 0.001 mm. 